This application is the national phase under 35 U.S.C. xc2xa7371 of PCT International Application No. PCT/JP 00/07239 which has an-International filing date of Oct. 18, 2000, which designated the United States of America and was not published in English.
1. Technical Field
The present invention relates to an acoustic wave apparatus for propagating acoustic waves, used for the circuit of a communication equipment, an electronic device or the like.
2. Background Art
Heretofore, in such an acoustic wave apparatus in which a piezoelectric substrate containing lithium tantalate (LiTaO3, referred to as LT hereinafter) has been used, the cut angle xcex8 of the LT substrate has been set equal to 36xc2x0. This setting was a result of the calculation that if an electrode was formed on the surface of such a substrate, and the substrate surface was electrically short-circuited, the amount of propagation loss would be reduced to nearly a value of zero.
However, such calculation was made by assuming the establishment of an ideal state where the electrode had no thickness. Consequently, in the actual acoustic wave apparatus comprising an electrode having thickness, there was a possibility that a condition for reducing the amount of propagation loss to a minimum may be different. In addition, the calculation was made by examining the case where the entire surface of the substrate was covered with the electrode. Consequently, in the acoustic wave apparatus comprising electrode fingers cyclically arrayed as in the case of an SAW filter, there was a possibility that a condition for reducing the amount of propagation loss to a minimum might be different.
Thus, in Japanese Patent Application Laid-Open No. 1997-167936 (referred to as a document 1, hereinafter), a condition for reducing the amount of propagation loss to a minimum is examined by taking into consideration the thickness of a grating electrode formed on the surface of the LT substrate. FIG. 1 shows the result of calculating the amount of propagation loss in a ladder surface acoustic wave filter of the document 1 shown in FIG. 7. In the drawing, an ordinate indicates the amount of loss made when a surface acoustic wave (referred to as SAW, hereinafter) is propagated per wavelength (xcex), i.e., the amount of loss per wavelength (dB/xcex). An abscissa indicates a standardized electrode thickness (h/xcex), where the thickness h of the electrode is standardized based on the wavelength xcex of SAW.
FIG. 1 shows the case where an LT crystal X-axis direction is set as a SAW propagation direction, a surface perpendicular to a xe2x80x9cxcex8-rotated Yxe2x80x9d axis obtained by rotating a crystal Y axis by xcex8 around the crystal X axis, is set as a substrate surface, and a cut angle xcex8 is set in the range of 36xc2x0 to 46xc2x0. The LT substrate having the surface perpendicular to the xe2x80x9cxcex8-rotated Yxe2x80x9d axis set as its surface and the crystal X-axis direction set as the SAW propagation direction is represented by xcex8-rotated Y-cut X-propagation lithium tantalate, abbreviated to xcex8YX-LT, or xcex8YX-LiTaO3. In many cases, the electrode is made of aluminum (Al) or an alloy mainly containing Al.
As shown in FIG. 1, if a standardized electrode thickness (h/xcex) is xcex8, the amount of loss per wavelength (dB/xcex) is minimum when a cut angle xcex8 is about 36xc2x0. This result coincides with that of the conventional calculation, i.e., if the ideal state of the electrode having no thickness is established, the amount of propagation loss is reduced to nearly a value of zero when a cut angle xcex8 is 36xc2x0.
In addition, as shown in FIG. 1, if a cut angle xcex8 is 40xc2x0, the amount of loss per wavelength (dB/xcex) is minimum when a standardized electrode thickness (h/xcex) is about 0.05. If a cut angle xcex8 is 42xc2x0, the amount of loss per wavelength (dB/xcex) is minimum when a standardized electrode thickness (h/xcex) is about 0.075. Accordingly, in the SAW device realized by setting the standardized electrode thickness (h/xcex) in a range above 0.05, a cut angle xcex8 for reducing the amount of propagation loss to a minimum resides in a range above 40xc2x0.
As apparent from the foregoing discussion made with reference to FIG. 1, it is possible to reduce the amount of propagation loss to a minimum by selecting the proper combination of a standardized electrode thickness (h/xcex) with a cut angle xcex8. As a result, the insertion loss of the SAW device can be reduced. Therefore, in recent years, the LT substrate having a cut angle xcex8 set equal to 42xc2x0 has been employed.
There are several kinds of acoustic waves. If a cut angle xcex8 is set in the range of about 36xc2x0 to 46xc2x0, and the direction of propagation is a crystal X axis, for example, a surface skimming bulk wave (SSBW), which is a bulk wave propagated along the surface of an LT substrate described in a document: pp. 158-165, xe2x80x9cJournal of Institute of Electronics and Communication Engineers of Japanxe2x80x9d, Vo 1. J67-C, No. 1, January 1984 (referred to as a document 2, hereinafter), and a leaky surface acoustic wave (LSAW) are propagated. In the present application, these waves are generically termed as SAW, except when the waves are distinguished from each other.
FIG. 2 is an upper surface view showing the constitution of the SAW filter, which is one type of an acoustic wave apparatus. In the drawing, a reference numeral 1 denotes an LT substrate made of a piezoelectric material; 3 an electrode finger; 4 a bonding pad; 5 an input side interdigital transducer (IDT) for performing electricxe2x80x94surface acoustic wave energy conversion; 6 an output side IDT for performing surface acoustic wavexe2x80x94electric energy conversion; 7 an input terminal; and an 8 an output terminal. W 0 represents a maximum value of the length of a portion intersected by the electrode finger 3.
FIG. 3 is a sectional view of the SAW filter shown in FIG. 2. In the drawing, a code w represents a width of the electrode finger 3; p an arraying cycle of electrode fingers 3; and h a thickness of the electrode finger 3.
Next, the operation of the SAW filter will be described.
An electric signal applied to the input terminal 7 forms an electric field at the intersection of each electrode finger 3 of the input side IDT 5. In this case, as the LT substrate 1 is made of the piezoelectric material, the electric field causes distortion. If the input signal has a frequency f, the strain that has been generated is vibrated at the frequency f, converting the signal into SAW. This SAW is then transmitted in a direction perpendicular to the electrode finger 3. At the output side IDT 6, the SAW is converted back into the electric signal. The conversion of the electric signal into the SAW, and the conversion of the SAW into the electric signal are reversible to each other.
If a cut angle xcex8 is about 36xc2x0, and the propagation direction of the SAW is in a crystal X-axis direction, as described in the document 2, the displacement component of the SAW has a direction parallel to the electrode finger 3, and the surface of the LT substrate 1. Such a displacement component depends on the cut angle xcex8 of the cut surface of the LT substrate 1 and the propagation direction of the SAW.
The SAW excited by the input side IDT 5 is propagated toward the output side IDT 6. However, if there is propagation loss in the LT substrate 1, the power of the SAW having reached the output side IDT 6 is smaller than that of the SAW immediately after its excitation by the input side IDT 5. The amount of the loss is approximately equal to a value obtained by multiplying a distance between the centers of the input side IDT 5 and the output side IDT 6 standardized based on the wavelength xcex of the SAW by an attenuation constant xcex1.
Thus, assuming that the distances of the input side IDT 5 and the output side IDT 6 are equal to each other, as the amount of propagation loss in the LT substrate 1 is increased, the amount of insertion loss for the SAW filter is larger. As described in a document: pp. 56 to 81, xe2x80x9cSurface Acoustic Wave Engineeringxe2x80x9d, Institute of Electronics and Communication Engineers of Japan, issued by Corona Inc., November 1983, since the wavelength xcex of the SAW is double the arraying cycle p of the electrode fingers 3, the amount of loss generated following propagation is approximately equal to a value, which is obtained by multiplying a numerical value half an average value of the numbers of electrode fingers 3 constituting the input side IDT 5 and the output side IDT 6 by an attenuation constant xcex1.
For example, as shown in FIG. 2, assuming that each of the input side IDT 5 and the output side IDT 6 has 7 electrode fingers 3, and the input side IDT 5 and the output side IDT 6 are disposed close to each other, the amount of loss generated following propagation is equal to a value, which is about 3 to 4 times larger than the attenuation constant xcex1. As an example, if an attenuation constant xcex1 is 0.02 (dB/xcex), then the amount of loss following propagation takes a value set in the range of 0.06 to 0.08 dB.
As apparent from the foregoing, in order to realize a low-loss SAW device, it is important to use an LT substrate 1 having a small amount of propagation loss. Heretofore, in the acoustic wave apparatus of the foregoing type, a cut angle xcex8 set in a range above 36xc2x0 has been employed.
As described above, the propagation loss greatly affects the insertion loss of the SAW filter. However, the propagation loss is not the only factor that affects the insertion loss of the SAW filter. As material constants for representing the characteristics of the LT substrate 1, in addition to the propagation loss, there are an electromechanical coupling coefficient k2 regarding conversion efficiency between the electric signal and the SAW, a capacitance C0 regarding the impedance of the input or output side IDT 5 or 6, the propagation velocity Vs of the SAW, and so on. Among these constants, the electromechanical coupling coefficient k2 is particularly important, because it decides the insertion loss or the pass band width of the SAW filter.
For the acoustic wave apparatus using a pure surface acoustic wave bringing about no propagation loss in principle, such as a Rayleigh wave, Bleustein-Gulyaev-Shimizu (BGS) wave or the like, optimal designing conditions were known. However, for the acoustic wave apparatus using LSAW or SSBW, no specific conditions were known.
As described above, the conventional acoustic wave apparatus of the foregoing type has been used under the condition for minimizing the propagation loss. However, since the electromechanical coupling coefficient k2 for greatly affecting the characteristics of the acoustic wave apparatus has not been used under any optimal conditions, deterioration has inevitably occurred in the insertion loss or the band width of the acoustic wave apparatus.
The present invention was made to solve the foregoing problems, and it is an object of the invention to provide an acoustic wave apparatus with lower loss characteristics and wider band than the conventional acoustic wave apparatus of the foregoing type.
In accordance with the present invention, there is provided an acoustic wave apparatus, comprising: a piezoelectric substrate mainly containing lithium tantalate; and an interdigital transducer including a conductor formed on the substrate. In this case, a surface rotated in the range of 34xc2x0 to 41xc2x0 from a crystal Y axis around a crystal X axis of the lithium tantalate is set as the surface of the substrate, a standardized electrode thickness (h/xcex) obtained by standardizing a thickness h of an electrode finger constituting the interdigital transducer by a wavelength xcex of a surface acoustic wave is set in the range of 0.01 to 0.05, and a duty ratio (w/p) of the electrode finger decided based on a width w and an arraying cycle p of the electrode finger is set to the value ranging from 0.6 to just below 1.0.
Thus, an acoustic wave apparatus with lower loss characteristics and wider band characteristics than the conventional apparatus can be realized.
In accordance with the invention, there is provided an acoustic wave apparatus, comprising: a piezoelectric substrate mainly containing lithium tantalate; and an interdigital transducer including a conductor formed on the substrate. In this case, a surface rotated in the range of 35xc2x0 to 42xc2x0 from a crystal Y axis around a crystal X axis of the lithium tantalate is set as the surface of the substrate, a standardized electrode thickness (h/xcex) obtained by standardizing a thickness h of an electrode finger constituting the interdigital transducer by a wavelength xcex of a surface acoustic wave is set in the range of 0.05 to 0.075, and a duty ratio (w/p) of the electrode finger decided based on a width w and an arraying cycle p of the electrode finger is set to the value ranging from 0.6 to just below 1.0.
Thus, an acoustic wave apparatus with lower loss characteristics and wider band characteristics than the conventional apparatus can be realized.
In accordance with the invention, there is provided an acoustic wave apparatus, comprising: a piezoelectric substrate mainly containing lithium tantalate; and an interdigital transducer including a conductor formed on the substrate. In this case, a surface rotated in the range of 36xc2x0 to 43xc2x0 from a crystal Y axis around a crystal X axis of the lithium tantalate is set as a surface of the substrate, a standardized electrode thickness (h/xcex) obtained by standardizing a thickness h of an electrode finger constituting the interdigital transducer by a wavelength xcex of a surface acoustic wave is set in the range of 0.075 to 0.1, and a duty ratio (w/p) of the electrode finger decided based on a width w and an arraying cycle p of the electrode finger is set to the value ranging from 0.6 to just below 1.0.
Thus, an acoustic wave apparatus with lower loss characteristics and wider band than the conventional apparatus can be realized.
In accordance with the invention, there is provided an acoustic wave apparatus, comprising: a piezoelectric substrate mainly containing lithium tantalate; an interdigital transducer including a conductor formed on the substrate; and a reflector including a conductor formed on the substrate. In this case, a surface rotated in the range of 34xc2x0 to 41xc2x0 from a crystal Y axis around a crystal X axis of the lithium tantalate is set as a surface of the substrate, a standardized electrode thickness (h/xcex) obtained by standardizing a thickness h of an electrode finger constituting at least a part of the reflector by a wavelength xcex of a surface acoustic wave is set in the range of 0.01 to 0.05, and a duty ratio (w/p) of the electrode finger decided based on a width w and an arraying cycle p of the electrode finger is set to the value ranging from 0.6 to just below 1.0.
Thus, an acoustic wave apparatus with lower loss characteristics and wider band characteristics than the conventional apparatus can be realized.
In accordance with the invention, there is provided an acoustic wave apparatus, comprising: a piezoelectric substrate mainly containing lithium tantalate; an interdigital transducer including a conductor formed on the substrate; and a reflector including a conductor formed on the substrate. In this case, a surface rotated in the range of 35xc2x0 to 42xc2x0 from a crystal Y axis around a crystal X axis of the lithium tantalate is set as a surface of the substrate, a standardized electrode thickness (h/xcex) obtained by standardizing a thickness h of an electrode finger constituting at least a part of said reflector by a wavelength xcex of a surface acoustic wave is set in the range of 0.05 to 0.075, and a duty ratio (w/p) of the electrode finger decided based on a width w and an arraying cycle p of the electrode finger is set to the value ranging from 0.6 to just below 1.0.
Thus, an acoustic wave apparatus with lower loss characteristics and wider band characteristics than the conventional apparatus can be realized.
In accordance with the invention, there is provided an acoustic wave apparatus, comprising: a piezoelectric substrate mainly containing lithium tantalate; an interdigital transducer including a conductor formed on the substrate; and a reflector including a conductor formed on the substrate. In this case, a surface rotated in the range of 36xc2x0 to 43xc2x0 from a crystal Y axis around a crystal X axis of the lithium tantalate is set as a surface of the substrate, a standardized electrode thickness (h/xcex) obtained by standardizing a thickness h of an electrode finger constituting at least a part of the reflector by a wavelength xcex of a surface acoustic wave is set in the range of 0.075 to 0.1, and a duty ratio (w/p) of the electrode finger decided based on a width w and an arraying cycle p of the electrode finger is set to the value ranging from 0.6 to just below 1.0.
Thus, an acoustic wave apparatus with lower loss characteristics and wider band characteristics than the conventional apparatus can be realized.
In accordance with the invention, there is provided an acoustic wave apparatus, comprising: a piezoelectric substrate mainly containing lithium tantalate; and an interdigital transducer including a conductor formed on the substrate. In this case, a surface rotated in the range of 34xc2x0 to 41xc2x0 from a crystal Y axis around a crystal X axis of the lithium tantalate is set as a surface of the substrate, a standardized electrode thickness (h/xcex) obtained by standardizing a thickness h of an electrode finger constituting a part of the interdigital transducer by a wavelength xcex of a surface acoustic wave is set in the range of 0.01 to 0.05, and a duty ratio (w/p) of the electrode finger decided based on a width w and an arraying cycle p of the electrode finger is set to the value ranging from 0.6 to just below 1.0.
Thus, an acoustic wave apparatus with lower loss characteristics and wider band characteristics than the conventional apparatus can be realized.
In accordance with the invention, there is provided an acoustic wave apparatus, comprising: a piezoelectric substrate mainly containing lithium tantalate; and an interdigital transducer including a conductor formed on the substrate. In this case, a surface rotated in the range of 35xc2x0 to 42xc2x0 from a crystal Y axis around a crystal X axis of the lithium tantalate is set as a surface of the substrate, a standardized electrode thickness (h/xcex) obtained by standardizing a thickness h of an electrode finger constituting a part of the interdigital transducer by a wavelength xcex of a surface acoustic wave is set in the range of 0.05 to 0.75, and a duty ratio (w/p) of the electrode finger decided based on a width w and an arraying cycle p of the electrode finger is set to the value ranging from 0.6 to just below 1.0.
Thus, an acoustic wave apparatus with lower loss characteristics and wider band characteristics than the conventional apparatus can be realized.
In accordance with the invention, there is provided an acoustic wave apparatus, comprising: a piezoelectric substrate mainly containing lithium tantalate; and an interdigital transducer including a conductor formed on the substrate. In this case, a surface rotated in the range of 36xc2x0 to 43xc2x0 from a crystal Y axis around a crystal X axis of the lithium tantalate is set as a surface of the substrate, a standardized electrode thickness (h/xcex) obtained by standardizing a thickness h of an electrode finger constituting a part of the interdigital transducer by a wavelength xcex of a surface acoustic wave is set in the range of 0.075 to 0.1, and a duty ratio (w/p) of the electrode finger decided based on a width w and an arraying cycle p of the electrode finger is set to the value ranging from 0.6 to just below 1.0.
Thus, an acoustic wave apparatus with lower loss characteristics and wider band characteristics than the conventional apparatus can be realized.
In accordance with the invention, there is provided an acoustic wave apparatus, comprising: a piezoelectric substrate mainly containing lithium tantalate; an interdigital transducer including a conductor formed on the substrate; and a reflector including a conductor formed on the substrate. In this case, a surface rotated in the range of 34xc2x0 to 41xc2x0 from a crystal Y axis around a crystal X axis of the lithium tantalate is set as a surface of the substrate, a standardized electrode thickness (h/xcex) obtained by standardizing a thickness h of a part of an electrode finger constituting a part of the reflector by a wavelength xcex of a surface acoustic wave is set in the range of 0.01 to 0.05, and a duty ratio (w/p) of a part of the electrode finger decided based on a width w and an arraying cycle p of a part of the electrode finger is set to the value ranging from 0.6 to just below 1.0.
Thus, an acoustic wave apparatus with lower loss characteristics and wider band characteristics than the conventional apparatus can be realized.
In accordance with the invention, there is provided an acoustic wave apparatus, comprising: a piezoelectric substrate mainly containing lithium tantalate; an interdigital transducer including a conductor formed on the substrate; and a reflector including a conductor formed on the substrate. In this case, a surface rotated in the range of 35xc2x0 to 42xc2x0 from a crystal Y axis around a crystal X axis of the lithium tantalate is set as a surface of the substrate, a standardized electrode thickness (h/xcex) obtained by standardizing a thickness h of a part of an electrode finger constituting a part of the reflector by a wavelength xcex of a surface acoustic wave is set in the range of 0.05 to 0.075, and a duty ratio (w/p) of a part of the electrode finger decided based on a width w and an arranging cycle of a part of the electrode finger is set to the value ranging from 0.6 to just below 1.0.
Thus, an acoustic wave apparatus with lower loss characteristics and wider band characteristics than the conventional apparatus can be realized.
In accordance with the invention, there is provided an acoustic wave apparatus, comprising: a piezoelectric substrate mainly containing lithium tantalate; an interdigital transducer including a conductor formed on the substrate; and a reflector including a conductor formed on the substrate. In this case, a surface rotated in the range of 36xc2x0 to 43xc2x0 from a crystal Y axis around a crystal X axis of the lithium tantalate is set as a surface of the substrate, a standardized electrode thickness (h/xcex) obtained by standardizing a thickness h of a part of an electrode finger constituting a part of the reflector by a wavelength xcex of a surface acoustic wave is set in the range of 0.075 to 0.1, and a duty ratio (w/p) of a part of the electrode finger decided based on a width w and an arraying cycle p of a part of the electrode finger is set to the value ranging from 0.6 to just below 1.0.
Thus, an acoustic wave apparatus with lower loss characteristics and wider band characteristics than the conventional apparatus can be realized.